1. Field of the Invention
The present invention relates in general to an industrial robot for use in various jobs such as, for example, material handling, part assembling, welding and the like. More particularly, it relates to a manipulation arm mechanism for such an industrial robot capable of moving a working head such as a handling gripper, an assembling tool, a welding unit or the like, along straight and circular paths for performing any of the various jobs.
2. Description of the Prior Art
In a known manipulation arm mechanism for an industrial robot, as shown in FIG. 1, a working head 1 such as, for example, a handling gripper, is rotatable about each of an axis L1 and an oblique axis L2 and thus, has a working range covering a hemisphere whose axis is in axial alignment with the axis L1. That is, first to third shafts 3-5 have a common axis L1, and the axis of the working head 1 is in axial alignment with the common axis L1 when the working head 1 is oriented toward the forward direction, as shown therein. This arrangement advantageously makes the preparation of a motion control program for the robot easy. However, the co-axial arrangement of the axes of the first to third shafts 3-5 causes the working range of the working head 1 about the axis which perpendicularly intersects the oblique axis L2 in the area where the rear casing meets the front casing to be limited to 180-degrees at most.
Another manipulation arm mechanism of the same type as above has also been known, wherein the working range of a working head about the oblique axis L2 is broadened up to 240-degrees. However, if an attempt is made to enlarge the angle (.theta.1) which the oblique axis L2 makes with the rotational axis of the working head, then the result will be that the working head 1 is brought into the interference with the rear casing 6. Accordingly, it is impossible for said another manipulation are mechanism to cover a working range which is broader than 240 degrees.
Furthermore, in each of the aforementioned manipulation arm mechanisms, reduction gear trains are used to transmit rotations from drive motors respectively to the first to third shafts 3-5, and motion transmitting gear trains are used to transmit rotation from the second and third shafts 4 and 5 respectively to a front casing 7 and the working head 1. A backlash involved in each of these gear trains makes it difficult to precisely position the working head 1. Particularly, where a first shaft co-axially carries a second shaft therein which in turn, co-axially carries a third shaft therein as is true in each of the aforementioned prior art mechanisms, the second and third shafts can not be radially moved relative respectively to the first and second shafts, thus making it impossible to eliminate the backlash involved in each of the gear trains.